A molecular spectrometer is an instrument wherein a solid, liquid, and/or gaseous sample is illuminated, often with non-visible light, such as light in the infrared region of the spectrum. The light reflected from, scattered by, and/or transmitted through the sample is then captured and analyzed to reveal information about the characteristics of the sample. As an example, a sample may be illuminated with infrared light having known intensity across a range of wavelengths, and the light from the specimen can then be captured for comparison to the illuminating light. Review of the captured spectra (i.e., light intensity vs. wavelength data) can illustrate the wavelengths at which the illuminating light was absorbed by the sample, which in turn can yield information about the chemical bonds present in the sample, and thus its composition and other characteristics.
To explain the foregoing arrangement in greater detail, the light from the sample is generally focused onto an input aperture of a spectrograph, a subcomponent of the spectrometer which generates the desired spectral measurements. The spectrograph itself generally includes a monochromator, a device which separates the received light into its component wavelengths, and a photosensitive detector (or detector array) which receives at least some of the light at the component wavelengths and measures the light intensity at these wavelengths. The spectrograph input aperture is usually a slit or hole, with a narrower or smaller aperture size resulting in better differentiation between wavelengths, but at the same time leading to decreased light to the detector and greater noise in intensity measurements. The size of the aperture additionally affects the size of the area or volume of the sample being analyzed, since it is effectively the image of the sample as viewed through the aperture (and ultimately projected onto the detector) which generates the spectral readings. For example, a pinhole (small circular hole) is often used as an aperture for confocal spectrometry arrangements, wherein readings are to be limited to a very specific area on the sample (the focus).
Owing to the tradeoffs involved with selection of an aperture size, it is often desirable to allow a spectrometer user the ability to vary aperture sizes, as by moving opposing masks together (or apart) to allow variation of the width of a slit defined therebetween, or by rotating or translating a mask having differently-sized pinholes or slits thereon to situate an aperture of the desired size and configuration at the entrance of the spectrograph. The problem with these arrangements is that for best results, the aperture must be precisely situated at the focus of the beam carrying the sample image. It is common for arrangements such as rotating/translating masks to eventually fall out of alignment with the focus after extended use, leading to maintenance burdens and/or degraded measurement quality. Further, while reliable variable-width slits can be constructed using the aforementioned opposing respaceable masks, it is difficult to similarly form a resizable pinhole aperture. One could conceivably situate one mask pair immediately adjacent another mask pair, with one mask pair allowing slit size variation in one direction and the other mask pair allowing slit size variation in a perpendicular direction so that the two effectively combine to form a variably-sized “pinhole,” but it is difficult to construct a reliable mechanism for doing so without creating an undesirably bulky mask which at least partially obstructs some of the desired light. In other words, it is difficult to construct such an arrangement without having the masks extend for an undesirable distance in front of and behind the focal point of the image beam. It would therefore be useful to have available spectrometry arrangements which allow variation of aperture sizes and/or configurations, while still providing a reliable mechanism for resizing which is resistant to misalignment problems.